A method of reducing the x-ray dose in an x-ray system

ABSTRACT

Disclosed herein is a method of reducing the x-ray dose of a patient in an x-ray system, comprising defining a region of interest of the patient, obtaining at least two tracking images of a tracking element taken with at least one camera having a known positional relationship relative to an x-ray source and/or sensor, determining any movement of the tracking element between the acquisition of at least two tracking images, adjusting the collimator of the x-ray source to compensate for any movement of the tracking element between the acquisition of the at least two tracking images, providing that the field of exposure of the x-ray source is confined to the region of interest and obtaining at least one x-ray image of the region of interest after the adjustment of the collimator.

FIELD OF THE INVENTION

This invention generally relates to a system and method for controllingthe collimator in a medical imaging device. More particularly, theinvention relates to the tracking of patient movements during imageacquisition in a medical imaging device, in particular in Cone BeamComputed Tomography (CBCT) scanners, and using this information todirect the x-rays to a defined Region of Interest (ROI).

BACKGROUND OF THE INVENTION

Computed tomography, particularly x-ray computed tomography (CT), is awidely used volumetric imaging principle. In general terms, a radiationsource and a radiation-sensitive image sensor are arranged on a line,with the subject of the examination positioned in between. The subjectattenuates the radiation. The source-detector arrangement is typicallymoved into several positions, often on a circle or segment thereof,around the subject of the examination, and images are taken at everyposition. The spatial, volumetric distribution of the attenuationcoefficient within the subject can then be reconstructed from allimages, for example using the filtered back projection algorithm,generating a 3D digital model of the subject. Often, the image sensor isa 2D sensor, such as in cone beam computed tomography (CBCT). Inmedicine, x-ray CT scanners are valuable non-invasive diagnosticdevices.

A major concern related to the use of CT scanners in medicine isradiation dose. Accordingly, a large body of research has focused onvolumetric reconstruction algorithms that exploit the image data in anoptimal way, allowing fewer images to be taken, or a lower dose perimage, for a given quality of the reconstruction. While filtered backprojection is a direct algorithm, many refined algorithms are iterativeones. Because the volumetric reconstruction problem is ill-posed,various regularization approaches have been suggested, e.g., totalvariation. Maximum-likelihood estimation has also been proposed, forexample with a prior based on material assumptions. Several proposedreconstruction algorithms contain some of the above elements, or all ofthem.

Another way to lower the needed dose in a CBCT system is to make surethe patient does not move during image acquisition. This is because fora given needed accuracy, the signal-to-noise ratio will be greater whenthe patient does not move. Also, when the patient moves, motionartifacts such as for example streaks and aliasing may deteriorate theimage quality. Therefore, in general, the image quality will be betterwhen patient movement is kept to a minimum.

In prior art CBCT systems, various forms of head fixation devices havebeen employed to keep the patient fixated during the x-ray recording.These systems all have the goal of minimizing effects from motion blurand patient movement, thereby achieving a higher accuracy of the finalimages. However, all these systems have the disadvantage that it may beuncomfortable for the patient to be fixated for the duration of thescan, in particular for patients that may suffer from claustrophobia. Ittherefore remains a problem to achieve a high accuracy of CBCT imageswithout having to fixate the patient.

Yet another way to lower the used dose in a CBCT system is to accuratelydefine a region of interest (ROI). Many existing CT scanners allow theselection of a region of interest, but the selection is madeschematically, and in order to ascertain that the relevant volume iscovered, the region of interest suggested by the system is often largerthan necessary for a particular patient. Also, since there is a riskthat the patient moves during x-ray image acquisition, the chosen ROI isoften larger than ideally necessary, in order to make certain that thewhole ROI is covered in a single scan. It may be useful if the CTscanner allowed realization of the adjustment of the region of interestby for example four independent collimator shutters for the top, bottom,left, and right sides of the beam.

SUMMARY OF THE INVENTION

In one aspect there is disclosed a method of obtaining medical images ofa patient using a medical imaging device, the method comprising:

-   -   defining a region of interest of the patient;    -   obtaining at least two tracking images of a tracking element        taken with at least one camera having a known positional        relationship relative to a radiation source and/or sensor;    -   determining any movement of the tracking element between the        acquisition of at least two tracking images;    -   adjusting the medical imaging device based on the determined        movement of the tracking element between the acquisition of the        at least two tracking images so that the radiation passes        through the region of interest; and    -   obtaining at least one medical image of the region of interest        after the adjustment of the medical imaging device.

In this way, it is insured that even if the patient moves during theacquisition of medical images, the radiation is confined to the regionof interest, thereby making it possible to for example define a smallerregion of interest.

In another aspect there is disclosed a method of obtaining one or morex-ray images of a patient, the method comprising:

-   -   defining a region of interest of the patient;    -   obtaining at least two tracking images of a tracking element        taken with at least one camera having a known positional        relationship relative to an x-ray source and/or sensor;    -   determining any movement of the tracking element between the        acquisition of at least two tracking images;    -   adjusting the collimator of the x-ray source to compensate for        any movement of the tracking element between the acquisition of        the at least two tracking images, providing that the field of        exposure of the x-ray source is confined to the region of        interest; and    -   obtaining at least one x-ray image of the region of interest        after the adjustment of the collimator.

In X-ray optics, a collimator is a device that filters a stream of raysso that only those traveling parallel to a specified direction areallowed through. Collimators are used in X-ray optics because it is notyet possible to focus radiation with such short wavelengths into animage through the use of lenses as is routine with electromagneticradiation at optical or near-optical wavelengths. Without a collimator,rays from all directions will be recorded; for example, a ray that haspassed through the top of the specimen but happens to be travelling in adownwards direction may be recorded at the bottom of the plate,resulting in a blurred image.

Accordingly, it is thus possible to compensate for any unwanted movementthe patient makes during acquisition of the x-ray images. The movementof the tracking element corresponds to a movement of the patient, andsince there is a feedback between the determination of the movement ofthe tracking element and the collimator, it is possible to focus thex-rays on the region of interest, even if the patient has moved.

In some embodiments a scout image is taken of the patient. The scoutimage may be taken with a lower resolution/image quality using the x-raysource and sensor, or the scout image may be taken using a surfaceimaging device, for example using a face scanner, an intra-oral scannerand/or a surface contour laser scanner, and the region of interest isdefined using the scout image.

In this way, the region of interest chosen will correspond to the exactgeometry of the patient, rather than using a generic or standardgeometry to define the region of interest.

In some embodiments, the predefined information of the tracking elementcomprises at least one fiducial marker, such as a plurality of fiducialmarkers in a predefined pattern, size, shape and/or colour.

When the placement, size, shape and/or colour of the fiducial markersare already known with very high accuracy before any images are taken,it is possible to determine with very high accuracy the movement of thetracking element between images. In prior art systems, landmarks on thepatient have been used to track any movement of the patient. However,this is not as accurate as using a tracking element, for examplecomprising fiducial markers placed on the tracking element with a veryhigh and known placement, because unlike in the current disclosure, thelandmarks have to first be determined or marked by an operator or bycomputer software. This means that the exact position of the landmarkswill not be as accurate as using a tracking element. Also, when takingfor example a series of chest x-rays, the breathing of the person willaffect the relative positions of the landmarks, so that this in itselfwill lead to a less accurate result.

In some embodiments, the tracking images and the x-ray images are timestamped using the same clock.

One way to correlate the movement of the patient with the x-ray data, isto map the movement of the tracking element in time with the recordingof the x-ray data. In principle, the cameras recording the trackingelement and the x-ray sensor could be run using two separate processorswith each their own clock. However, in this case, the two clocks wouldhave to be synchronized in order to be able to map exactly the movementof the patient with the medical imaging data. A simpler solution is tohave both the cameras and the x-ray device run using the same clock.This can be accomplished for example by running the cameras and thex-ray device from the same computer processor. The computer processormay be a stand-alone desktop or laptop computer or any other type ofcomputer means, or it may be integrated in the scanner.

In some embodiments, determining the position and orientation of thetracking element at each time stamp comprises:

-   -   recognizing a plurality of the individual fiducial markers in        each tracking image;    -   obtaining a digital representation in a database of the known        predefined pattern and/or shape of the fiducial markers;    -   recognizing the pattern of the fiducial markers in each image to        achieve a best fit to the known predefined pattern of the        fiducial markers on the tracking element from each tracking        image.

In order to determine the orientation and position of the trackingelement, image analysis algorithms can be used. For example, if thefiducial markers are in the form of dots of a known size, the algorithmscan be used to detect where there are dots and what size they have. Themethod used may for example be principal component analysis (PCA),although other methods are also possible and known to the person skilledin the art.

Since the fiducial markers have a known size, shape and/or predefinedpattern on the tracking element, once the size, shape and position ofeach found dot is determined, a mask comprising the known predefinedpattern of the fiducial markers can by loaded from a database, beoverlayed on the tracking image, and the fit of the tracking image tothe mask can be determined, thereby finding the orientation and positionof the tracking element.

In some embodiments there may be more than one camera, such as twocameras or three cameras for recording the movement of the trackingelement. The reason for this, is that if only one camera is used, it isdifficult to unambiguously determine how far away from the camera thefiducial marker is. If two cameras are used, it is difficult tounambiguously determine the position of the tracking element in adirection that is parallel to a line connecting the two cameras. If, onthe other hand, three cameras are used, possibly but not necessarily,placed for example at the points of an equilateral triangle, theposition of the tracking element in all three dimensions can beunambiguously determined.

Determining the position and orientation of the tracking element usingthree cameras, can be accomplished for example by having the images fromthe three cameras time stamped so that at each time t, there are threeimages taken of the element, recognizing the fiducial markers in eachimage, determining a best fit to the known predefined pattern of thefiducial markers on the tracking element in each image, determining theposition and orientation of the tracking element in each of the threeimages of the tracking element at each time stamp, and computing aweighted average of the position and orientation of the tracking elementfrom the three images.

In some embodiments, determining the position and orientation of thetracking element at each time stamp comprises:

-   -   recognizing a plurality of the individual fiducial markers in        each tracking image;    -   using classification of the indices of the fiducial markers; and    -   matching the known pattern of the fiducial markers on the        tracking element to the pattern of the fiducial markers on the        tracking image using the classification of the indices of the        fiducial markers.

Matching the known pattern of the fiducial markers may for example beaccomplished using a computer device, where the tracking images areloaded, and the fiducial markers are recognized and/or segmented in thetracking images. Then, the position of the fiducial markers in thetracking image are indexed, and the index of the fiducial markers in thetracking image are compared to the known index of the fiducial markerson the tracking element. Since the distance between the fiducial markerson the tracking element is known, the distances between the fiducialmarkers in the tracking images can be compared to the known distances,and known mathematical algorithms can be used to determine the positionand rotation of the tracking element in the tracking images.

In some embodiments, the camera position and rotation of each camera iscalibrated or determined;

-   -   the intrinsic parameters such as the focal length, skew,        principal point and lens distortion are calibrated or determined        for each camera;    -   the tracking images from the three cameras are acquired        simultaneously such that at each time t, there are three images        taken of the tracking element;    -   the fiducial markers are recognized in each tracking image and        the position of each fiducial marker is determined directly in        the camera co-ordinate frame;    -   the position and/or orientation of the tracking element from the        three images is determined using a cost function to minimise the        difference in the determined position of the fiducial markers in        each of the tracking images.

Since extrinsic parameters of the cameras are known (i.e. the positionand rotation of the cameras with relation to the medical imagingdevice), and the fiducial markers are recognized in each image and theposition of the fiducial markers are determined directly in theco-ordinate frame of the camera, the determination of the position androtation of the tracking element relative to the medical imaging devicewill be more accurate.

In some embodiments, the tracking element is attached to a headband,which can be placed on the patient's head. It is an advantage if theheadband is adjustable, since it should be possible to securely attachthe headband to patients with different head sizes such as children andadults, without any risk of the headband moving during the exposuretime.

The tracking element may have only one fiducial marker, but preferablyshould have a plurality of fiducial markers on its surface, for examplein the form of dots or circles. There may be any number of fiducialmarkers, for example more than 10, more than 100, more than 200 or morethan 400 dots. Preferably there should be enough dots to make it simpleto find the position and size of the dots, but not so many that it wouldtake too much processing time.

In some embodiments, there are asymmetrical features on the trackingelement or the tracking element itself is asymmetrical. In principle, itis possible to determine the position and orientation of the trackingelement even if the fiducial markers are all placed in a completelysymmetrical pattern. In this case, it would be assumed that the trackingelement has moved the shortest possible distance that is consistent withthe pattern of the fiducial markers, between each time stamp. However,if the fiducial markers are placed asymmetrically, or if the trackingelement itself is asymmetrical, there is no ambiguity in when overlayingthe mask of the known predefined pattern with the image of the trackingelement.

The adjustment of the collimator should take place substantially in realtime during acquisition of the x-ray images. In this way, any movementof the patient will be reflected in the adjustment of the collimator.The adjustment of the collimator may comprise tilting the collimator,moving the collimator in a horizontal and/or vertical direction and/orany other direction, and/or changing any other characteristics of thecollimator such as the size of the opening of the collimator.

There are many different designs of collimators, and any collimator canbe used with this invention. The collimator may for example be a set offour lead plates that can be individually adjusted, to change the sizeof the opening and the direction of the x-rays, or it may be a grid ofrods that can be adjusted to create a similar effect, or any otherdesign that is capable of directing the x-rays.

The inventive concept of this specification can be used advantageouslyin any medical imaging device where it is important that the patient isstill during imaging, such as standard x-ray, magnetic resonanceimaging, positron emission tomography, etc. However, it is particularlyuseful in CBCT systems where it is very important to get a very highaccuracy of the scan, and where it is important to achieve a low dose ofx-ray exposure.

In some embodiments, the fiducial markers are in the form of circulardots. Dots or circles are simple geometrical features, that are easilyrecognized by computer algorithms.

In some embodiments, the x-ray images are combined to form a digitalmedical model.

In some embodiments, the system may include a mouthpiece for helping thepatient stay still during the exposure. The mouthpiece may be in theform of a plate attached to the medical imaging device, and configuredto allow the patient to bite onto the plate.

The tracking element in this specification can be made from any materialsuch as plastic, glass, metal or the like. It is, however important thatthe tracking element is made out of a material that is substantiallyrigid, so that the known pattern of fiducial markers will not bedistorted over time.

In some embodiments, the tracking element is made of coated glass, andthe fiducial markers are printed on the surface of the glass. Thismaterial is both rigid, and it is relatively simple to etch or printfiducial markers on the surface of the glass with high accuracy.

In some embodiments, disclosed is a method of reducing the x-ray dose ofa patient in an x-ray system, the method comprising:

-   -   defining a region of interest of the patient;    -   obtaining a first tracking image of a tracking element taken        with at least one camera having a known positional relationship        relative to an x-ray source and/or sensor, said tracking images        depicting at least a part of the tracking element;    -   obtaining a second tracking image of the tracking element taken        with the at least one camera;    -   determining any movement of the tracking element between the        acquisition of the first and second tracking images;    -   adjusting the collimator of the x-ray system to compensate for        any movement of the tracking element between the acquisition of        the first and second tracking images providing that the field of        exposure of the x-ray source is confined to the region of        interest.

Since the collimator is adjusted to compensate for any movement of thepatient, the region of interest can be defined more narrowly than ifthere was no compensation for the movement of the patient. Therefore,the effective x-ray dose the patient receives will be less than if therewas no collimator adjustment.

In some embodiments, the x-ray system is configured to obtain one of apanoramic x-ray image, a cephalometric image, or any other type of2-dimensional x-ray image.

In some embodiments, the x-ray system is configured to obtain a3-dimensional digital model of at least a part of the patient, such asfor example a CBCT scan.

In this way, the use of the feedback between the tracking element andthe collimator of the x-ray source can be used to reduce the dose and/orraise the accuracy of any x-ray image, whether it is a 2D-image or a 3Ddigital model.

In another aspect, disclosed is a method of controlling the region ofinterest of a patient imaged using a medical imaging device, the methodcomprising:

defining a region of interest of the patient;obtaining a first tracking image of a tracking element taken with atleast one camera having a known positional relationship relative to aradiation source and/or sensor, said tracking image depicting at least apart of the tracking element;obtaining a second tracking image of the tracking element taken with theat least one camera;determining any movement of the tracking element between the acquisitionof the first and second tracking images;adjusting the position of the radiation source and/or radiation sensorof the medical imaging system to compensate for any movement of thetracking element between the acquisition of the first and secondtracking images providing that the field of exposure of the radiationsource is confined to the region of interest.

In this way it is possible to confine the field of exposure, for exampleof an x-ray source to the region of interest on the patient withoutadjusting the collimator, but instead by changing the geometry of themedical imaging device. For example, in the case of a CBCT system, thex-ray source and x-ray sensor are placed on a ring capable of rotatingaround the patient. In this way, if the patient has moved as determinedfrom the position of the tracking element in a series of trackingimages, the ring containing the x-ray source and sensor can be rotatedto better align the x-rays with the region of interest. Of course,depending on the movement of the patient and the exact geometry of thesetup of the medical imaging device, the x-ray source may not beconfined completely within the region of interest of the patient.However, it will in most cases be better confined to the region ofinterest using this setup.

In some embodiments, the medical imaging device is a CBCT scanner, andthe at least one medical image are x-ray images defining a panoramictrajectory, wherein the method further comprises:

-   -   adjusting the CBCT scanner to follow the determined panoramic        trajectory based on the determined movement from the tracking        element.

In some cases, it is desired to obtain a panoramic x-ray. A panoramicx-ray is a two-dimensional image that captures the entire oral area inone image, including teeth, upper and lower jaws, and the surroundingstructures and tissues. When using a CBCT system, a panoramic trajectoryis defined, so that the patient is exposed to the x-rays in apredetermined path that allows for capturing the panoramic x-ray. If thepatient moves during this time, the panoramic trajectory may no longerbe in correspondence with the actual trajectory, meaning that theresulting panoramic image may be blurred. In this case, the panoramictrajectory can be corrected based on the determined movement of thetracking element. This can be accomplished by adjusting the positionand/or exposure of the x-ray source and/or sensor based on thedetermined movement of the tracking element In another aspect, disclosedherein is a system for obtaining medical images of a patient, the systemcomprising:

-   -   a medical imaging device comprising;    -   one or more tracking image cameras configured to take tracking        images of a tracking element;    -   a computer device comprising a micro processor and a computer        readable medium;    -   a visual display unit;    -   input means for controlling the medical imaging device;        wherein the computer device is configured to adjust the medical        imaging device in response to determined movement of the        tracking element.

This system comprises the means for performing the methods according tothe previous aspects and embodiments.

In some embodiments, the computer device is configured to adjust themedical imaging device by adjusting a collimator of the medical imagingdevice.

In this way, the computer device can adjust for example the beam of anx-ray source, by adjusting the collimator of the x-ray machine.

In some embodiments, the computer device is configured to adjust themedical imaging device by changing the geometry of the medical imagingdevice.

In this case, instead of adjusting a collimator of the medical imagingdevice, the geometry of the medical imaging device can be changed. Forexample, in a CBCT scanner, the ring on which the x-ray sensor and x-raysource are attached, the computer device can adjust the position of thering, so that the x-ray sensor and/or x-ray source are moved relative tothe patient.

In the context of this specification, the term medical imaging devicecovers any device capable of taking medical images of a patient, such asx-ray, magnetic resonance imaging, computed tomography, positronemission tomography, cone beam computed tomography etc.

In the context of this specification, the term tracking element shouldbe understood to mean any device that can be attached to the patient forthe purpose of determining their movement, and should therefore not beconfined to mean only a flat rectangular piece of metal or plastic. Inprinciple, the form of the tracking element could be for examplecircular, semi-circular, pyramidal, triangular, or any other shape. Thetracking element could even be a complex three dimensional shape, wherethe shape of the tracking element itself is used as the fiducialmarkers.

Embodiments

-   1. A method of obtaining medical images of a patient using a medical    imaging device, the method comprising:    -   defining a region of interest of the patient;    -   obtaining at least two tracking images of a tracking element        taken with at least one camera having a known positional        relationship relative to a radiation source and/or sensor;    -   determining any movement of the tracking element between the        acquisition of at least two tracking images;    -   adjusting the medical imaging device based on the determined        movement of the tracking element between the acquisition of the        at least two tracking images so that the radiation passes        through the region of interest; and    -   obtaining at least one medical image of the region of interest        after the adjustment of the medical imaging device.-   2. The method according to embodiment 1, wherein adjusting the    medical imaging device based on the determined movement of the    tracking element between the acquisition of the at least two    tracking images comprises adjusting the collimator of the radiation    source to compensate for any movement of the tracking element    between the acquisition of the at least two tracking images,    providing that the field of exposure of the radiation is confined to    the region of interest.-   3. The method according to embodiment 1, wherein adjusting the    medical imaging device based on the determined movement of the    tracking element between the acquisition of the at least two    tracking images comprises changing the geometry of the medical    imaging device, so that either the radiation sensor and/or source is    moved relative to the region of interest, based on the determined    movement of the tracking element.-   4. The method according any of the preceding embodiments, wherein    the medical imaging device comprises an x-ray source and sensor, and    a scout image is taken with a lower resolution/image quality using    the x-ray source and sensor, and the region of interest is defined    using the scout image.-   5. The method according to any of the preceding embodiments, wherein    a scout image is taken using a face scanner, an intra-oral scanner    and/or a surface contour laser scanner, and the region of interest    is defined using the scout image.-   6. The method according to any of the preceding embodiments, wherein    the tracking element comprises a predefined geometry and/or    predefined information.-   7. The method according to any of the preceding embodiments, wherein    determining any movement of the tracking element between the    acquisition of at least two tracking images comprises;    -   recognizing a plurality of fiducial markers in each tracking        image;    -   obtaining a digital representation in a database of the known        predefined pattern and/or shape of the fiducial markers;    -   recognizing the pattern of the fiducial markers in each image to        achieve a best fit to the known predefined pattern of the        fiducial markers on the tracking element from each tracking        image.-   8. The method according to any of embodiments 1-7, wherein    determining any movement of the tracking element between the    acquisition of at least two tracking images comprises:    -   recognizing a plurality of the individual fiducial markers in        each tracking image;    -   using classification of the indices of the fiducial markers; and    -   matching the known pattern of the fiducial markers on the        tracking element to the pattern of the fiducial markers on the        tracking image using the classification of the indices of the        fiducial markers.-   9. The method according to any of the preceding embodiments, wherein    the x-ray images are combined to make a digital medical model.-   10. The method according to any of the preceding embodiments,    wherein the medical imaging device is a cone beam computed    tomography scanner.-   11. The method according to embodiment 10, wherein the at least one    medical image are x-ray images defining a panoramic trajectory,    wherein the method further comprises:    -   adjusting the CBCT scanner to follow the determined panoramic        trajectory based on the determined movement from the tracking        element.-   12. The method according to embodiment any of the preceding    embodiments, wherein the x-ray system is configured to take one of a    panoramic x-ray image, a cephalometric image, or any other type of    2-dimensional x-ray image or a CBCT scan of the patient.-   13. A system for obtaining medical images of a patient, the system    comprising:    -   a medical imaging device comprising;    -   one or more tracking image cameras configure to take tracking        images of a tracking element;    -   a computer device comprising a micro processor and a computer        readable medium;    -   a visual display unit;    -   input means for controlling the medical imaging device;        wherein the computer device is configured to adjust the medical        imaging device in response to determined movement of the        tracking element.-   14. The system according to embodiment 13, wherein the computer    device is configured to adjust the medical imaging device by    adjusting a collimator of the medical imaging device.-   15. The system according to embodiment 13, wherein the computer    device is configured to adjust the medical imaging device by    changing the geometry of the medical imaging device.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and/or additional objects, features and advantages of thepresent invention, will be further described by the followingillustrative and non-limiting detailed description of embodiments of thepresent invention, with reference to the appended drawings, wherein:

FIG. 1 shows a flow chart of a method according to an embodiment of thisinvention.

FIG. 2 shows a flow chart of a method according to an embodiment of thisinvention.

FIG. 3 shows a tracking element according to an embodiment of thisinvention.

FIG. 4 shows a CBCT scanning system according to an embodiment of thisinvention.

FIG. 5 shows a stylized view of a collimator according to an embodimentof this invention.

DETAILED DESCRIPTION

An embodiment of the method disclosed herein is shown in FIG. 1.

In step 101, a scout image is taken of the patient using the x-raysource and sensor, typically at a lower resolution and/or image qualitythan what is used in the subsequent exposure. Lower resolution and/orimage quality may comprise for example using a lower x-ray dose, if thescout image is taken using x-rays. While the scout image is taken, ahead tracking system is started. The head tracking system comprises atleast one camera, which is configured to take images of a trackingelement attached to the head of the patient. The position andorientation of the tracking element is determined in step 102, and atsubsequent times, tracking images are taken of the tracking element, andthe position and orientation of the tracking element is determined.Based on this determination, the movement of the tracking element, andtherefore the movement of the patient, may be determined substantiallycontinuously. In step 103, a region of interest is defined using thescout image. The scout image may for example be displayed on a touchscreen or on a computer that has controls for the x-ray scanner, and theregion of interest may be defined interactively by the operator, or itmay be suggested automatically by the system. Once the region ofinterest is defined, the resolution of the x-ray scanner may be set to ahigher resolution/image quality, if this is needed for the final x-rayimages. In step 104, the collimator controlling the path of the x-raysis dynamically adjusted based on the determined movement of the patient,so that the x-rays are confined to expose the region of interest. Inthis way, even if the patient moves during the x-ray image generation,which could comprise one or more of a CBCT scan, a panoramic image, acephalometric image or any other type of x-ray, only the region ofinterest will be subject to x-ray exposure. Therefore, the region ofinterest can be set to be as small as possible, giving the patient alower x-ray dose. In step 105, one or more x-ray images at the higherresolution/image quality is taken of the patient using the x-rayscanner. Since the region of interest was defined using the scout image,and the tracking element is attached to the patient during subsequentexposures, any movement of the tracking element can be correlated with amovement of the region of interest.

FIG. 2 shows a flow chart representing an embodiment of the methoddisclosed herein. In step 201, a tracking element, here in the form of aplate, with at least one fiducial marker is attached to the head of apatient. The fiducial markers may be any shape, for example a circle,triangle, ellipse, or any other geometrical shape. In step 202, a scoutimage is taken using either the x-ray source, a face scanner, a videocamera, or any other imaging device. If the scout image is taken usingthe x-ray source, the scout image will typically be taken with a lowerresolution/image quality than the final x-ray images. In step 203 a, themedical imaging device acquires medical images of the patient.Concurrently with step 203 a, in step 203 b, tracking images of theplate are taken using one or more cameras that are placed in a knownspatial relationship with the medical imaging source and sensor. Thecameras may be integrated into the medical imaging device, or they maybe a separate system. In step 204, the position, size and tilt of thefiducial markers is determined. This can for example be done by usingprincipal component analysis. If, for example the fiducial markers arein the form of circular dots, when there is an angle between a normalvector of the plate and a linear axis between the plate and the camera,the circular dots will look slightly deformed in the tracking image. Inthis case, principal component analysis can be used to determine whetherwhat is observed in the image is a dot, and where the center of the dotis located. In step 205, a mask of the known predefined pattern of thefiducial markers is loaded from a database, and compared with thedetermined pattern of fiducial markers in each tracking image. Thiscomparison can be done using any method known in the art. This allowsthe position and orientation of the plate to be determined. It may beadvantageous to determine the orientation of the midpoint of the plate,since this will allow the highest accuracy. However, the position andorientation of any point on the plate may be used, for example thecorner of the plate.

If there is more than one camera, a tracking image from each camera willbe taken at each time t. Each of these tracking images will then have adetermined position and orientation of the plate at each time t. Theposition and orientation determined from each tracking image at time tmay be slightly different because of the particular geometry of thesituation, for example one camera may have a more acute angle towardsthe plate than another. The determined position and orientation fromeach tracking image at time t may then be combined into a singledetermined position and orientation. This combination can for example bedone by performing a weighted average of the position and orientationmeasurement from each tracking image at time t.

The weighted average can for example be computed by starting with thefound position and orientation of the tracking element from one image,determining the difference between this starting position and theposition and orientation of the tracking element in each of the othertwo images, and iteratively adjusting the starting position andorientation of the tracking element to an adjusted position andorientation, until the combined error or difference between the positionand orientation of the tracking element in each image and the adjustedposition and orientation is minimized.

Alternatively, the starting position and orientation of the trackingelement could be a standard default position and orientation, and thedifference between this standard position and orientation and theposition and orientation determined in each of the three images can becomputed. Then the starting position and orientation of the trackingelement can be iteratively adjusted until the combined error ordifference between the position and orientation of the tracking elementin each image and the adjusted starting position is minimized.

Therefore the accuracy of the determined position and orientation of theplate will be better when more than one camera is used.

An alternative approach to the comparison step 205 may be accomplishedas follows. Instead of having a database containing a mask of the knownpredefined pattern of the fiducial pattern or markers, there may insteadbe a classification of the indices of each of the fiducial markers, asexplained in relation to FIG. 3. In this way, the 3D position andorientation of the element is then found such that the classificationindices of the known pattern is matched with the determined indices ofthe fiducial markers on the image sensor after projecting. Here it isimportant to note that the field of view of each camera, should be largeenough to unambiguously determine which part of the element is in theimage. In the case of more than one camera, there may be ambiguities asto the exact position and orientation of the element as determined fromthe tracking images taken with different cameras. In this case, a costfunction may be used, so that the position and orientation determinationis optimized using information from all cameras.

In step 206, the movement of the plate between different times t isdetermined, and the determined movement of the plate is correlated to amovement of the region of interest. Since the positional relationshipbetween the cameras and the medical imaging source and sensor is known,any movement of the plate can be directly translated into acorresponding movement of the patient, and therefore the region ofinterest.

In step 207, any determined movement of the region of interest is usedto adjust the collimator so that the x-rays converge on the region ofinterest. Alternatively, instead of adjusting the collimator, the x-raysource and or sensor may be adjusted or moved based on the determinedmovement of the region of interest. This will typically be the case if alarger movement of the patient has occurred, for example if the movementis 1 cm or more. However, no matter the value of the actual determinedmovement of the patient, the collimator and/or the x-ray sensor and/orthe x-ray source may be moved or adjusted.

In FIG. 3, a tracking element 1 according to embodiments of thisdisclosure is shown. The tracking element has the form of a rectangularplate, made of a rigid material. The plate has a plurality of fiducialmarkers 2, in a predetermined pattern, layout or configuration. Thepattern should be known to a very high degree of accuracy, so thatmatching subsequent tracking images taken of the plate, can be matchedwith a mask of the same pattern saved in a database. In CBCT systemstoday, typical accuracy is in the range 75-350 microns at the moment.Therefore, the accuracy of the known placement of each fiducial markershould at least be within this range in order to achieve a higheraccuracy in the digital medical model. Of course, the higher theaccuracy of the placement of the fiducial markers, the more the accuracywill be improved.

Each fiducial marker may be classified using a classification index. Forexample, the fiducial marker closes to one corner could be defined ashaving the index (0,0), the next one in the same row could have theindex (0,1) and in general the fiducial markers could have an indexdefined as (i,j), with I going from 0 to n, and j going from 0 to m. Inthis way, the fiducial markers will have a known classification index,which can then be compared to tracking images to match the actualpattern of the fiducial markers on the element to the fiducial markersin the tracking images.

However, when the system is used for example for a cephalometric imageor a panoramic image or any 2-dimensional x-ray, a lesser accuracy maybe sufficient. For example, if a patient moves several millimeters orcentimeters, any accuracy better than the movement of the patient willyield a more accurate final x-ray image/model. The plate may alsocomprise an asymmetrical feature 3. This will make it easier forcomputer algorithms to unambiguously match the pattern from the databaseto the tracking images, and therefrom derive the actual position andorientation of the tracking element in each tracking image. In the casewhere the fiducial markers are classified using a classification index,the asymmetrical feature will mean that it will be easier to make surethat each tracking camera has a view of the element wherein the positionand orientation of the element in the field of view of the camera canmore easily be unambiguously derived. That is, once the fiducial markershave been segmented in the tracking images, for example using PCA, theycan be classified according to the classification index. If, on theother hand, the field of view of the tracking camera only covered anambiguous subset of the fiducial markers, it would be impossible tounambiguously derive the position and orientation of the element in thetracking image.

The tracking element may be made of any rigid material such as plastic,metal or glass. When using coated glass for the element, it is easy toprint or etch the fiducial markers onto or into the surface of theelement.

Although illustrated here as a rigid plate on which the fiducial markersare printed or etched, the tracking element may also for example be aplate with holes, with lights placed underneath the holes, so that theposition of the lights can be picked up by a sensor. The lights couldfor instance use infrared wavelengths, and the sensor could be aninfrared sensor. Another option could be to have an active plate wherelights are placed on the surface of the plate, and the position of theselights could be picked up by a sensor. For example, the light could beLED lights.

Turning now to FIG. 4, a system according to an aspect of thisdisclosure is shown. The system comprises a medical imaging device inthe form of a CBCT scanner 10, where the CBCT scanner comprises a sensor11, and a radiation source 12. The sensor and/or the radiation sourceare able to turn substantially around a full circle around the patient'shead. The system may also comprise a chin rest 13 for the patient torest his/her chin. The system may also include a face scanner (notshown), the face scanner configured to record a 3D model of thepatient's face. The system further comprises a tracking element 1, hereshown as a plate attachable to the patient's head. Also comprised in thesystem is one or more cameras, for example located inside the ring 15.The cameras should be mounted with a known geometrical relationship tothe sensor 11 and radiation source 12. Often, this will be near or inthe center of the ring 15, since the patient will usually be positionedunderneath the center of the ring 15. The cameras are configured to beused to take tracking images of the tracking element 1 simultaneouslywith the CBCT scanner taking x-ray images. In front of, or integrated inthe radiation source is a collimator, which can be adjusted to focus orconverge or point the x-rays in a certain direction.

FIG. 5 shows a stylized view of the adjustable collimator 17 asdisclosed herein. The x-ray source 12 provides x-rays 18, and thecollimator 17 is fully adjustable, so that the x-rays 18 can be directedtowards the region of interest 16. The collimator can have any form, forexample it can be composed of four independent shutters controlling thetop, bottom, left and right of the x-ray beam.

FIG. 6 shows a schematic of a system according to an embodiment of theinvention. The system 600 comprises a computer device 602 comprising acomputer readable medium 604 and a microprocessor 603. The systemfurther comprises a visual display unit 605, input means for enteringdata and activating digital buttons visualized on the visual displayunit 605. In some embodiments as shown here, the input means may be acomputer keyboard 606 and a computer mouse 607. The visual display unit605 can be a computer screen, or a tablet computer, or any other digitaldisplay unit. In some cases when the visual display unit is for examplea tablet computer, the input means may be the touch screen of the tabletcomputer.

The computer device 602 is capable of obtaining medical images recordedwith one or more medical imaging devices 601 a and tracking imagesrecorded by one or more cameras 601 b. The obtained medical images andtracking images can be stored in the computer readable medium 604 andprovided to the processor 603. In some embodiments system 600 may beconfigured for allowing an operator to control the medical imagingdevice using the computer device 602. The controls may displayeddigitally on the visual display unit 605, and the user may control themedical imaging device, as well as the tracking cameras using thecomputer keyboard 606 and computer mouse 607.

The system may comprise a unit 608 for transmitting the medical images,the tracking images and/or the digital medical model via the internet,for example to a cloud storage.

The medical imaging device 601 a may be for example a CBCT unit locatedfor example at a dentist office.

Although some embodiments have been described and shown in detail, theinvention is not restricted to them, but may also be embodied in otherways within the scope of the subject matter defined in the followingclaims. In particular, it is to be understood that other embodiments maybe utilized and structural and functional modifications may be madewithout departing from the scope of the present invention.

In device claims enumerating several means, several of these means canbe embodied by one and the same item of hardware. The mere fact thatcertain measures are recited in mutually different dependent claims ordescribed in different embodiments does not indicate that a combinationof these measures cannot be used to advantage.

A claim may refer to any of the preceding claims, and “any” isunderstood to mean “any one or more” of the preceding claims.

The term “obtaining” as used in this specification may refer tophysically acquiring for example medical images using a medical imagingdevice, but it may also refer for example to loading into a computer animage or a digital representation previously acquired.

It should be emphasized that the term “comprises/comprising” when usedin this specification is taken to specify the presence of statedfeatures, integers, steps or components but does not preclude thepresence or addition of one or more other features, integers, steps,components or groups thereof.

Some features of the method described above and in the following may beimplemented in software and carried out on a data processing system orother processing means caused by the execution of computer-executableinstructions. The instructions may be program code means loaded in amemory, such as a RAM, from a storage medium or from another computervia a computer network. Alternatively, the described features may beimplemented by hardwired circuitry instead of software or in combinationwith software.

1. A method of obtaining medical images of a patient using a medicalimaging device, the method comprising: defining a region of interest ofthe patient; obtaining at least two tracking images of a trackingelement taken with at least one camera having a known positionalrelationship relative to a radiation source and/or sensor; determiningany movement of the tracking element between the acquisition of at leasttwo tracking images; adjusting the medical imaging device based on thedetermined movement of the tracking element between the acquisition ofthe at least two tracking images so that the radiation passes throughthe region of interest; and obtaining at least one medical image of theregion of interest after the adjustment of the medical imaging device.2. The method according to claim 1, wherein adjusting the medicalimaging device based on the determined movement of the tracking elementbetween the acquisition of the at least two tracking images comprisesadjusting the collimator of the radiation source to compensate for anymovement of the tracking element between the acquisition of the at leasttwo tracking images, providing that the field of exposure of theradiation is confined to the region of interest.
 3. The method accordingto claim 1, wherein adjusting the medical imaging device based on thedetermined movement of the tracking element between the acquisition ofthe at least two tracking images comprises changing the geometry of themedical imaging device, so that either the radiation sensor and/orsource is moved relative to the region of interest, based on thedetermined movement of the tracking element.
 4. The method according anyof the preceding claims, wherein the medical imaging device comprises anx-ray source and sensor, and a scout image is taken with a lowerresolution/image quality using the x-ray source and sensor, and theregion of interest is defined using the scout image.
 5. The methodaccording to any of the preceding claims, wherein a scout image is takenusing a face scanner, an intra-oral scanner and/or a surface contourlaser scanner, and the region of interest is defined using the scoutimage.
 6. The method according to any of the preceding claims, whereinthe tracking element comprises a predefined geometry and/or predefinedinformation.
 7. The method according to any of the preceding claims,wherein determining any movement of the tracking element between theacquisition of at least two tracking images comprises; recognizing aplurality of fiducial markers in each tracking image; obtaining adigital representation in a database of the known predefined patternand/or shape of the fiducial markers; recognizing the pattern of thefiducial markers in each image to achieve a best fit to the knownpredefined pattern of the fiducial markers on the tracking element fromeach tracking image.
 8. The method according to any of claims 1-7,wherein determining any movement of the tracking element between theacquisition of at least two tracking images comprises: recognizing aplurality of the individual fiducial markers in each tracking image;using classification of the indices of the fiducial markers; andmatching the known pattern of the fiducial markers on the trackingelement to the pattern of the fiducial markers on the tracking imageusing the classification of the indices of the fiducial markers.
 9. Themethod according to any of the preceding claims, wherein the x-rayimages are combined to make a digital medical model.
 10. The methodaccording to any of the preceding claims, wherein the medical imagingdevice is a cone beam computed tomography scanner.
 11. The methodaccording to claim 10, wherein the at least one medical image are x-rayimages defining a panoramic trajectory, wherein the method furthercomprises: adjusting the CBCT scanner to follow the determined panoramictrajectory based on the determined movement from the tracking element.12. The method according to any of the preceding claims, wherein thex-ray system is configured to take one of a panoramic x-ray image, acephalometric image, or any other type of 2-dimensional x-ray image or aCBCT scan of the patient.
 13. A system for obtaining medical images of apatient, the system comprising: a medical imaging device; one or moretracking image cameras configured to take tracking images of a trackingelement; a computer device comprising a micro processor and a computerreadable medium; a visual display unit; input means for controlling themedical imaging device; wherein the computer device is configured toadjust the medical imaging device in response to determined movement ofthe tracking element.
 14. The system according claim 13, wherein thecomputer device is configured to adjust the medical imaging device byadjusting a collimator of the medical imaging device.
 15. The systemaccording to claim 13, wherein the computer device is configured toadjust the medical imaging device by changing the geometry of themedical imaging device.